This study involves patients with Systemic Lupus Erthematosus who have concurrent kidney involvement. The purpose is to evaluate if a new experimental drug, BG9588 (an anti-CD4OL antibody), used in conjunction with steroids, reduces the level of abnormally excreted proteins and maintains normal kidney function. We will attempt to slowly decrease steroids during the study. Up to 30 subjects will be enrolled at 14 sites, including Columbia where the goal is 3 enrollees. Subjects will be considered as having completed the study if they have received seven 30-minute IV infusions of study drug, and completed 12 visits on study including follow-up visits 84 days after the last treatment cycle.